# Observation of parity-time symmetry breaking in a single spin system

## TL;DR

This paper reports the first experimental observation of parity-time symmetry breaking in a quantum system using a single NV center in diamond, demonstrating a novel method to realize and study non-Hermitian Hamiltonians in quantum mechanics.

## Contribution

The authors developed a new dilation method to embed non-Hermitian $ ext{PT}$ symmetric Hamiltonians into Hermitian systems, enabling experimental investigation in quantum systems.

## Key findings

- Observed $ ext{PT}$ symmetry breaking in a single spin system.
- Demonstrated state evolution from unbroken to broken $ ext{PT}$ symmetry.
- Established a universal method for simulating $ ext{PT}$ symmetric Hamiltonians in quantum systems.

## Abstract

A fundamental axiom of quantum mechanics requires the Hamiltonians to be Hermitian which guarantees real eigen-energies and probability conservation. However, a class of non-Hermitian Hamiltonians with Parity-Time ($\mathcal{PT}$) symmetry can still display entirely real spectra. The Hermiticity requirement may be replaced by $\mathcal{PT}$ symmetry to develop an alternative formulation of quantum mechanics. A series of experiments have been carried out with classical systems including optics, electronics, microwaves, mechanics and acoustics. However, there are few experiments to investigate $\mathcal{PT}$ symmetric physics in quantum systems.Here we report the first observation of the $\mathcal{PT}$ symmetry breaking in a single spin system. We have developed a novel method to dilate a general $\mathcal{PT}$ symmetric Hamiltonian into a Hermitian one, which can be realized in a practical quantum system.Then the state evolutions under $\mathcal{PT}$ symmetric Hamiltonians, which range from $\mathcal{PT}$ symmetric unbroken to broken regions, have been experimentally observed with a single nitrogen-vacancy (NV) center in diamond. Due to the universality of the dilation method, our result opens a door for further exploiting and understanding the physical properties of $\mathcal{PT}$ symmetric Hamiltonian in quantum systems.